1. Field of the Invention
The present invention relates to a character wheel type printing apparatus capable of erasing characters and, more particularly, to a printing apparatus where a speed of a print hammer in an erasing operation is higher than that in a print operation so as to improve removability of ink.
2. Description of Related Art
There has been conventionally known a character wheel type electronic typewriter capable of printing and erasing characters. These character wheel type electronic typewriters include a carriage having a print hammer, a character wheel, a print ribbon, a correction ribbon and drive mechanisms therefor.
In this type of electronic typewriter, individual motors drive a position shifting mechanism for shifting the position of a holder member having the print ribbon and the correction ribbon from a print position, where the print ribbon faces the print hammer in the print operation, to an erasure position, where the correction ribbon faces the print hammer in the erasing operation, a print hammer drive mechanism and a print ribbon taking-up mechanism, respectively.
The inventor of the present invention has proposed an electronic typewriter where only one motor disposed in a carriage unit drives the mechanism for shifting the position of the holder member, the print hammer drive mechanism, the print ribbon taking-up mechanism and a correction ribbon taking-up mechanism, so as to realize a reduced size of the carriage at a reduced manufacturing cost. Consequently, in the above-described electronic typewriter, a print cam for driving the print hammer is formed integrally with a lift cam for lifting the holder member up to the erasure position in a drive shaft of the motor, and the print hammer is driven via the print cam according to the forward rotation of the motor. The holder member is shifted in the erasure position via the lift cam according to the reverse rotation of the motor at a predetermined angle at the time of erasing characters, to allow the print hammer to erase the characters via the print cam according to the forward rotation.
Furthermore, in order to reduce strike noise caused by the print hammer in the print and erasing operations, it is effective to make a speed of the print hammer lower than an usual speed. For the purpose of compensating a smaller strike force due to the low speed, a platen must be pressed by the print hammer for a slight period after the stroke. Accordingly, although the strike noise of the print hammer will be reduced and the adhesiveness of ink will be enhanced, a recess of the character deeper than usual will be formed on print paper with application of a pressure of the print hammer in the print operation. In addition, the ink of the print ribbon will adhere to a portion around the recess because of the larger recess formed on the print paper.
In the electronic typewriter proposed by the inventor of the present invention as described above, the motor for driving the print hammer is controlled in an erasing operation in the same manner as in the print operation, with concomitant problems that the recess becomes much larger under pressure, that the ink adhering to the recess corresponding to the character can be removed while the ink adhering to the portion around the recess cannot be removed sufficiently, and so on.